Treatment of rubber



Patented July 7, 1953 TREATMENT OF RUBBER William McGillivray Morgan, Amer'sham, England, assignor to Monsanto Chemicals Limited, London, England, a British company No Drawing. Application March 6, 1952-, Serial No. 275,219. In Great Britain June 21-, 1948 14 Claims. 1 This invention relates to the processing of rubber, and to new vulcanization retarders;

The use of a vulcanization retarder in the coinpounding of rubber is common practice in order to reduce the tendency to scorch, whichcan be defined as partial. and premature vulcanization of the rubber stock taking place during the mixingop'eration in the course of which the. filler,- as for exam le carbon black, and vulcanization accelerator, among other substances'ar'e introduced, or during the shaping operation, extrusion or calendering for instance. The use of vulcanization retarders may, moreover, become of increasing importance with the greater use of the so-called furnace carbon blacks. In the process of mixing rubber carbon blacks made by the channel process (the so-called channel carbon blacks) are in wide use, especially in the tire industry, and carbon blacks made by the furnace process (known as furnace carbon blacks) are also in considerable use. However, there is a possibility that furnace carbon blacks may become even more widely used than they are at present, and it has been found that when using furnace blacks there is a greater tendency for scorching to take place during compounding. This tendency appears to increase with the fineness of the furnace black.-

I have now discovered a new series of vulcanization retarders, particularly useful in rubber stocks containing furnace carbon blacks, but also useful in other instances whether carbon blacks are present or not.

According to the invention there is employed as vulcanization retarder a salt of an aryl sufonic acid with a metal other than an alkali metal or cadmium. While free sin-ionic acids exert some retarding action they are corrosive to equipment and dangerous to handle. Furthermore, it has now been found that the salt of sulfonic acids actually are superior to the acids themselves. Zinc salts are especially effective. The optimum activity has been ob served with zinc salts of aryl sulfonic acids sub stituted in the nucleus by a long chain alkyl group as for example a hexyl, octyl nonyl; 'decyl' or dodecyl group. The zinc salt of dodec bem zene sulfonic acid is particularly valuable. How ever there are many others and examples are given below:

soar

SO 3M soar In the above compounds M represents a metal as defined and X represents a substituent group, for instance another sulfonic acid group, a h'ydrocarbon group (such as a methyl or other alkyl group, e. g. a long-chain group, a branched chain group, e. g. isopropylor isobutyl, or a waxresidue), hydroxyl or carboxyl, and moreover the acids may be further substituted, for instance by further groups of the type given as examples of X. Specific examples are zinc butyl naphthalene sulfonate, zinc decyl benzene sulfonate I dodecyl benzene sulfonate, removing the solvent by heating to 120 C., allowing the thick syrup thereby produced to set by cooling, and grinding the resultant brittle resin to a coarse powder.

Where in this specification dodecyl benzene sulfonic acid is referred to, this is the product obtained by chlorinating a kerosene containing a substantial quantity of dodecanes, condensing the product with] benzene by the Friedel-Crafts reaction, and sulfonating the condensate so formed. The zinc salt, for instance, can then be obtained by neutralizing the sulfonic' acid with zincoxide.

The invention is illustrated by the following" examples relating to the treatment of natural rubber, to which the invention primarily relates. It will be appreciatedhowever, that it is also useful in vulcanizing synthetic rubbers of the type which can be vulcanized with sulfur.

EXAMPLE 1 Zinc dodecyl benzene sulfonate in a quantity of two parts by weight was added during the mixing of the following stock, using a very fine particle size furnace carbon black:

Parts by weight Natural rubber (smoked sheets) 100 Carbon black 45 Zinc oxide Stearic acid 3 Rosin-paraffin oil softener 3 Sulfur 2 5 Mercaptc-benzothiazole 0.6

Portions of this stock and of one identical except that it was compounded without the addition of the zinc dodecyl benzene sulfonate were tested for scorching properties in the following manner.

Pellets in the form of small cylinders 1 cc. in volume and about 1.1 cm. in height were prepared by stamping out from the sheeted stocks and trimming to the stated volume, and were heated at 120 C. for different periods of time by placing them in an aluminum holder which was immersed in a fluid medium (glycerol) maintained at the required temperature under thermostatic control. After varying periods of time samples of both stocks were removed, cooled rapidly and stored under room temperature conditions for 18 hours, and their plasticity was then tested using a parallel plate plastometer at 70 C. Each pellet was preheated for. minutes at 70 C. and compressed under a load of 5 kilograms for 4 /2 minutes, and the thickness of the compressed pellet was recorded in hundredths of a millimeter.

Two pellets from each stock were tested for each period of heating, and the average thickness (to the nearest 5 hundredths of a millimeter) was termed the plasticity number.

The curves that can be drawn for the stocks 'by plotting the plasticity number against the time of heating at 120 C. can be compared directly to the base stock, the rate of rise in the plasticity number indicating the measure of scorching or precure. For convenience, as a rough guide in rapidly comparing results, the time for the plasticity number to reach a figure of above the lowest on the curve may be taken as a useful indication of the scorch time. The scorch time derived for the base stocks in thismanner was 18 and 11 minutes respectively and 26 minutes for the stock containing the zinc salt. From the scorch times thus derived the stocks can be compared by computing the percentage increase in scorch time over the base stocks. The superiority of zinc dodecyl benzene sulfcnate is apparent.

\ Percent Material Added to Base Stock 33555: for Scorch 3o Zinc dodecyl benzene suli'onate 44 Benzene sulbnic acid I 27 Dodecyl benzene sullonic acid none EXAlVIPLE 2 A similar series of experiments were carried out using different salts of dodecyl benzene sulfonic acid under similar conditions, except that the quantity of the retarder was one part by weight.

The following actual data were recorded: 45

T M I The following plasticity numbers were oba e tained:

Table III Base Stock Period of heating at 120 0., in minutes Base Stock plus the Zinc salt Period of heating at 120 0., in mics... 0 10 20 30 265 Base stock 260 325 495 000 285 Base stock plus the zinc salt 220 240 335 445 300 Base stock plus the calcium salt 255 275 435 550 310 Base stock plus the magnesium salt" 230 265 370 505 335 Base stock plus the copper salt 230 300 370 560 Base stock plus the barium San 23:; 200 no as A similar result was obtained when benzo- 'thiazyl disulfide was used as the vulcanization accelerator in place of mercaptobenzothiazole.

A similar series of experiments was carried out using benzene sulfonic acid and dodecyl benzene sulfonic acid respectively as the vulcanization retarder in place of the zinc dodecyl benzene sulfonate, but with the composition and conditions otherwise unaltered.

The values given below were obtained for the plasticity numbers. It will be appreciated that the difference between the figures obtained with the base stock and the corresponding figures in Table I is due to the fact that the two stocks were prepared on different occasions. Divergences of this kind are common in rubber technology, but the respective figures for the base stock provide proper comparison in each table.

Scorch time, mins.

Base stock 8 Base stock plus the zinc salt 13 Base stock plus the calcium salt' 12 Base stock plus the magnesium salt 11 Base stock plus the barium salt 11 EXAMPLE 3 Two similar series of experiments were carried out using the zinc salts of various aryl sulfonic acids, and the results given below were obtained. In Series A two parts by weight of the vulcanization retarder were used, and in Series B one part by weight, a separately prepared base stock being used in each, series.

The following values were obtained for the plasticity numbers:

Table IV Period. of heating at 120 0., in mins. 10 20 30 40 Series A:

sulfonate A. 220 250 285 365 445 Base stock plus zinc benzene sulfouate 235 255 300 395 470 Base stock plus zinc benzene di- (mate 230 255 315 390 480 Base stock plus zinc m-xylene sulionate s. c. 230 250 295 420 Base stock plus zinc naphthalene 2- sulfonate 225 245 325 425 l Base stock plus zinc p-cymene su 1ionate. 215 245 295 380 455 Base stock plus zinc isobutyl hy- A droxy diphenylsulfouate 225 265 310 435 580 Series B:

Base stock 335 370 510 710 Base stock plus zinc l-naphthol 4- suli'onate 295 345 425 595 Base stock plus zinc 2-naphtho1 4- sulfonate 335 370 485 605 Basestookpluszincphenolsul0uate 350 350 440 625 It will be noted that the zinc salts of aryl sulfonic acids functioned as efficient retarders in both series of tests. Furthermore, the data in Series A show that the zinc sulfonate substituted by a long chain alkyl group is outstanding. When the scorch times are determined by drawing plasticity curves that of the base stock is 11 minutes whereas that of the stock containing zinc dodecyl benzene sulfonate is slightly more than 13 minutes and the slope of the curve does not change appreciably before 20 minutes heating. After 13 minutes the plasticity numbers of the stocks containing the other zinc salts are all somewhat higher.

The method of preparing zinc dodecyl benzene sulfonate has already been referred toin this specification. The acids from which the other zinc salts were prepared were those formed by direct sulfonation of the organic compound in question, e. g. m-xylene in the case of m-xylene sulfonic acid and benzene sulfonic acid in the base of benzene disulfonic acid.

EXAMPLE 4.

While the new retarders are especially useful in furnace black stocks their retarding activity is by no means dependent upon the presence of furnace black as illustrated by this and the following three examples. Zinc dodecyl benzene sulfonate in a quantity of 0.5 part by weight was added during the mixing of the following stock:

Parts by weight Natural rubber (pale crepe) A"; 100 Precipitated whiting 30 Zinc oxide 5 Stearic acid s 1 Sulfur i A 2.5 Mercaptobenzothiazole 0.6

'Table V Base stool; Period of heating at G... m'minutes BaseStock plus, the inc, salt Table VI Base stock Period of heating at 100 0., in minutes Base Stock plus the zinc salt EXAIMPLE 6 A series of experiments similar to these described in Example 4 were carried out except'th'at instead of mercaptobenzothiazole there was used of tetramethyl thiuram disulfide, and the quanas vulcanization accelerator 0.375 part by weight tity of sulfur was reduced to 2.0 parts by weight.

The following results were obtained:

Table VII Base stock Period of heating at 100 0., in minutes Base Stock plus the zinc salt EXAMPLE 7 A series of experiments similar to those described in Example 6 were carried out except that instead of the tetramethyl thiuram disulfide there was used as vulcanization accelerator an equal amount of zinc diethyl dithiocarbamate.

Table VIII Basestock plus the zinc salt Period of heating at 100 0., in minutes Base Stock EXAMPLE 8 A base stock of the following composition was prepared using a very fine'particle size furnace carbon black:

Parts by weight Natural rubber (smoked sheets) 100 Carbon black 45 Zinc oxide Stearic acid 3 Rosin-paraflin oil softener 3 Sulfur 2.5 Benzothiazyl disulfide 0.45

To this base stock there was added 0.404 part of a double compound prepared from equimolecular proportions of diphenyl guanidine and zinc dodecyl benzene sulfonate as described earlier in this sepcifica ti on.

The stock was tested for scorching properties in the manner described in Example 1, with the following results:

Table IX Plasticity Period of heating at 120 (1., in mins. number EXAMPLE 9 A base stock of the following composition was prepared using a very fine particle size furnace carbon black:

Parts by weight To this base stock was added zinc dodecyl benzene sulfonate in the amounts indicated below and the resistance to scorch determined by means of a Mooney plastometer. This instrument is described by Melvin Mooney in Industrial and Engineering Chemistry, Analytical Edition, March 15, 1934, pages 147-151. The following actual data were recorded employing the large rotor at 121 C.

Table X Mooney Plasti city after Heating at Zinc salt added to Base 121 for Stool-z, Parts by weight 19mins. 20mins. 21mins. 22mins. 23mins.

The retarding action of the zinc salts is even more strikingly illustrated by Mooney plasticity readings at 135 C. At this temperature the slope of the plasticity curve for the base stock increased abruptly after 6 minutes heating whereas those of the stocks containing the zinc salt did not rise appreciably until after 9 minutes heating. The actual data are recorded below:

Table XI Mooney Plasticity after Heating at C. Zinc salt added to In addition the above described base and the stocks prepared by adding to it the zinc salt of dodecyl benzene sulfonic acid were cured in the usual manner by heating in a press for different periods of time at 126 C. The modulus and tensile properties are all comparable at full cure showing that no adverse affect on physical properties results from the presence of the zinc salt. However, the lower modulus of the initial cures as compared to the base is further illustrative of the increased processing safety afforded by incorporating a zinc sulfonate into a base stock which contains a delayed action accelerator.

Table XII Modulus of Zinc salt added to Cure Elasticity Tensile at Ultimate Base Stock, Time,in in lbs/in. Break, in Elongation, Parts by Weight mins. at 300% lbs/in. Percent Elongation EXAMPLE 10 A series of experiments was carried out employing the base stock of Example 1. The retarder was added to the base stock and resistance to scorch determined by means of a Mooney plastometer. The scorch point was taken as the time at which the plasticity curve begins to rise continuously. As illustrated by the data in Example 9 the readings remain fairly constant and then begin to rise rapidly, the rise showing that the stocks are beginning to cure. Two series of experiments were carried out at 135 C. the data for which are contained in the following table. It will be noted that in each series the zinc salts are more eflicient retarders than the free acids. However, the sodium and cadmium salts are either inactive or activate instead of retard cure.

Table XIII Scorch time, mins. at 135 0.

Material Added to Base Stock Amount Zinc dodecyl benzene sulionate Sodium dodecyl benzene sulionate Zinc benzene sulfonate Toluene suli'onic acid Zinc toluene sulfonate Cadmium dodecyl benzene sulionatc 9 the case of an acid substituted by a long chain alkyl group. However, at 121 C. retarding activity of benzene sulfonic acid was demonstrated although again the superiority of the zinc salts Additionally, the base stock and stocks containing 2.0 parts of a sulfonate as indicated were cured in the usual manner by heating in a press at 142 C. The modulus and tensile properties of the cured products are shown below:

Table XV Modulus of Ultimate Cure Elasticity Tensile at Material fiddled to Base Time, in g fig g Eilgiriga inmins a 300 o s. in.

Elongation percent None 15 l, 335 3, 465 580 Zinc benzene sulfonate 15 1, 166 3, 123 590 Zinc toluene sultonate 15 1, 080 2, 990 660 Zinc dodecyl benzene sulfonate 15 1, 080 3, 026 953 Sodium dodecyl benzene sulfonate 15 1, 465 3, 715 590 None 45 1, 950 3, 805 500 Zinc benzene sulfonate 45 l, 895 3, 910 535 Zinc toluene sulfonate.. 45 1, 915 3, 780 500 Sodium dodecyl benzene sulfonate 45 1, 890 3, 555 500 Zinc dodecyl benzene sulfonate 60 2, 033 3, 876 510 The initial cures show that the zinc salts all retard noticeably whereas the sodium salt shows activation if anything. However, it will be noted that at full cure the stocks containing the zinc salts give normal modulus and tensile properties. Furthermore these cure data are in agreement with the plasticity data of Table IV in showing that the presence of a long chain alkyl group in the zinc sulfonate nucleus provides stronger retardation in that a longer heating period is required to match the modulus and tensile properties of the base stock.

The superiority of the sulfonate saltsto the free sulfonic acids is surprising when it is considered that conversion of carboxylic acid retarders to salts usually lessens or entirely destroys the retarding activity. Furthermore the effect of sulfonates in furnace black stocks is to be contrasted to that of carboxylic acids and anhydrides. Typical carboxylic acids and anhydrides having a retarding effect in channel black stocks were evaluated and found to be ineffective in the presence of furnace black. Forexample,

0.4 part by Weight of a carboxylic acid was added to a base stock identical to that employed in Example 9 except that it contained 2 parts instead of 2.5 parts stearic acid and 2.5 parts instead of 2.0 parts of sulfur. Scorch was determined by means of a Mooney plastometer at 135 C. When evaluated in this manner stocks containing ortho-benzyl benzoic acid, maleic anhydride, monochloroacetic acid and naphthoylbenzoic acid showed no retardation whatsoever.

It will be understood that salts of other sulionic acids than those contained in the examples to illustrate the invention may be used. As other 10 examples there may be mentioned zinc salts of petroleum sulfonic acids and zinc cyclohexylsulfonate.

This application is a continuation-in-part of copending application Serial No. 97,078, filed June 3, 1949.

What is claimed is:

' 1. A process of vulcanizing a sulfur vulcaniz-' I able conjugated diene polymer rubber with sulfur and an organic accelerator in which there is employed as vulcanization retarder a small'amount of a member of the group consisting of zinc, calcium, magnesium, copper and barium saltsof an aryl sulfonic acid.

2. A process of vulcanizing natural rubber containing a furnace carbon black with sulfur and an organic accelerator in which there is employed as vulcanization retarder a small amount of a member of the group consisting of zinc, calcium, magnesium, copper and barium salts of an aryl sulfonic acid. 5

3. A process of vulcanizing a sulfur vulcanizable conjugated diene polymer rubber with sulfur and an organic accelerator in which there is employed as vulcanization retarder a zinc salt of an aryl sulfonic acid.

4. A process of vulcanizing natural rubber containing a furnace carbon black with sulfur and an organic accelerator in which there is employed as vulcanization retarder a small amount of a zinc salt of an aryl sulfonic acid substituted by an alkyl group containing at least '6 but not more than 12 carbon atoms.

5. A process of vulcanizing a sulfur vulcanizable conjugated diene polymer rubber containing a furnace carbon black with sulfur and an organic accelerator in which there is employed as vulcanization retarder a small amount of the zinc salt of dodecyl benzene sulfonic acid.

6. A process of vulcanizing a sulfur vulcaniz- 8. Rubber which has been vulcanized by the I process of claim 1.

9. Rubber whichchas been vulcanized by the process of claim 2.

References'Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,871,037 Cadwell "Aug. 9, 1932 2,419,512 Vesce Aug. 22, 1947 2,567,853 Morgan Sept. 11, 1951 

1. A PROCESS OF VULCANIZING A SULFUR VULCANIZABLE CONJUGATED DIENE POLYMER RUBBER WITH SULFUR AND AN ORGANIC ACCELERATOR IN WHICH THERE IS EMPLOYED AS VULCANIZATION RETARDER A SMALL AMOUNT OF A MEMBER OF THE GROUP CONSISTING OF ZINC, CALCIUM, MAGNESIUM, COPPER AND BARIUM SALTS OF AN ARYL SULFONIC ACID. 